The emergence of terahertz technology, with its wavelength range of about 3 mm to 30 μm, is generating significant industrial demand for sources such as pulsed lasers or continuous laser diodes, as well as imaging devices such as spectrometers, cameras, etc. Security, non-destructive testing, and research laboratory applications in particular are driving the development of high-performance terahertz imaging systems.
In response to such demand, terahertz imaging technologies are experiencing growth in two areas.
The first is in the growth of passive imaging systems, with measurement of ambient terahertz radiation. This is widely used in security. Whether for civil or military applications, or long-range vision in an opaque environment (smoke, fog, etc.), detection of weapons and prohibited products (in airports for example) has advanced the performance of cooled microbolometers for example.
The second is in a less advanced stage industrially, and concerns active imaging systems which make use of an external terahertz source. These are demonstrating growing potential in industrial applications. The contribution of spectroscopic analysis to imaging with femtosecond systems has become an important characteristic of the technology, with the development of pulsed laser sources covering a wide band of terahertz frequencies. This opens up a new range of applications for on-line non-destructive testing, although such imagers do not yet exist. In the laboratory, terahertz spectroscopic imaging exhibits the best sensitivity and the best dynamics. However, industrial applications are currently restrained by the fact that this type of imaging is achieved by scanning point by point (single-element detector), requiring a long acquisition time of several hours due to the length of the 2D scan to be performed and to the merging of the data to process the final image. The devices related to this technique are large and costly.
In order to exploit this potential, there has been significant activity in the development of a wide-band imaging camera with real-time data acquisition. A first commercial product exists that makes use of pyroelectric cameras, which are infrared motion detectors. The linearity, sensitivity and dynamics of pyroelectric cameras are not yet optimal for imaging, only for terahertz beam diagnostics.
The next generation of array detectors is currently still in the testing and development phase, primarily focusing on the use of thermal microbolometer cameras (sensitive within the 8-14 μm band) not yet adapted for the terahertz frequency domain.
Industrial and technological development of a real time terahertz imaging system is primarily limited today by the sensitivity and the signal-to-noise ratio (pyroelectric camera and microbolometer camera not being appropriate for the wavelength domain) for array detectors. The sensitivity can be increased using cryogenics, but the cost of obtaining a portable and compact system is high.
In another area of development, a first experimental attempt using a single-element thermal converter able to convert radiation into heat was conducted without success. The low sensitivity of the components used required high power terahertz sources (100 W) using a free-electron laser.
This type of technique is known, particularly from the document entitled “Visualization of Radiation from a High-Power Téra-hertz Free Electron Laser with a Thermosensitive Interferometer” by N. A. Vinoukorov et al., ISSN 1063-7842, Technical Physics, 2007, Vol. 52, no. 7, pages 911-919. This type of technique currently does not provide an absolute measurement, only a relative measurement of the radiation.